1. Field of the Invention
The present invention relates to a voltage controlled oscillator. More particularly, the invention relates to an open-loop (i.e., “unlocked”) oscillator and frequency generation method that can utilize changes in output frequency of the oscillator as a function of changes which affect the oscillator performance to determine transfer functions and apply those transfer functions to correct any skewing of the oscillator output.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art or conventional by virtue of their inclusion within this section.
Within nearly every electronic subsystem is some sort of generator that produces cyclical waveforms. The waveform generator is oftentimes referred to as an oscillator that produces regular oscillation signals. Depending on the application, an oscillator can be used simply as a source of regularly spaced pulses or as clock signals. Oscillators are oftentimes rated depending on their stability and accuracy, frequency adjustability (i.e., tunability), and power consumption.
There are numerous types of oscillators in the marketplace. A simple kind of oscillator is an RC relaxation oscillator. More complex and stable oscillators involve the more popular LC oscillator. While LC oscillators are more stable than RC oscillators, a crystal oscillator is generally more stable and accurate than LC oscillators.
Crystal oscillators use a piece of quartz (i.e., glass or silicon dioxide) that is cut and polished to vibrate at a specified frequency. Quartz is piezoelectric, wherein acoustic waves in the crystal are driven by an applied electric field and, in turn, can generate a voltage at the surface of the crystal. Because of the way the quartz is cut, crystal oscillators are formed as a bulk material separate and apart from the circuit in which the oscillator is coupled. Thus, crystal or ceramic oscillators (i.e., resonators) are not generally integrated into silicon and are therefore more expensive than the less accurate, less stable, LC oscillators.
It would be desirable to implement an LC oscillator because of its integration capability and lower manufacturing expense. However, most conventional LC oscillators suffer to a greater or lesser extent from drift introduced by environmental changes (hereinafter known as external events or “parameters”) such as temperature, power supply, and ground supply fluctuations, and variation in electrical components associated with the oscillating source. LC oscillators are also affected by internal parameters, such as the design characteristics of semiconductor layout and the overall fluctuations or changes from run-to-run in the fabrication or processing of the oscillator components.
One mechanism in which to minimize internal and external parameter fluctuations that cause the oscillating signal to drift is to implement the LC oscillator into a “locked” system. In a locked system, the voltage-controlled oscillator (VCO) of the LCVCO oscillator is placed within a closed loop. A signal driven from the VCO is fed back to and compared with a reference signal. One form of comparison is through a phase or frequency detector. The voltage proportional to the phase error is then applied to the VCO as part of the overall phase-locked loop (PLL).
While the closed loop PLL-based LCVCO system is relatively stable and accurate, the closed loop system is also complex and requires a reference signal most often derived from an oscillator utilizing quartz or other non-integrated resonator. In the case of a locked or unlocked system, an input voltage is applied to the VCO and, specifically, to the capacitor of the LCVCO. As stated above, the oscillating signal output is generally affected in either a linear or non-linear fashion based on external and internal parameters. However, the resulting drift must be accounted for in an unlocked system whereas a locked system eliminates the drift.
Therefore, it would be desirable to implement an oscillator that is fully integrated on a single monolithic substrate, and does not rely upon an external crystal resonator or the large and expensive conventional SAW resonators or MEMs devices. The desired system is preferably one that is a fully integrated, voltage-controlled oscillator (VCO) and, more preferably, an LCVCO.